The new macs can't be upgraded. You have to have everything installed from the jump. After that, it is what it is. A lot of the mac hardware is built right into the mother board. This allows for a faster computer, but makes it impossible to easily upgrade in the future.

The reason apple build non-upgradable hardware has nothing to do with speed, it has to do with $$$.

It's the same reason the iPhone doesn't have a uSD slot unlike most high end Android and Blackberry phones.

Why put a uSD slot in a phone when you can charge 10x per GB for onboard flash storage and have a built in artificial product limitation?